1. Field of the Invention
The present invention relates to a vehicle gear transmission including a main shaft having a predetermined number of drive gears, and a countershaft having a predetermined number of driven gears adapted to be respectively driven by the drive gears so as to set a predetermined number of shift positions. The gear transmission also includes a shifter engageable with the drive gears or the driven gears so that the main shaft rotationally drives the countershaft at any one of the shift positions.
2. Description of the Background Art
A conventional gear transmission is known to include a main shaft having a plurality of drive gears, a countershaft having a plurality of driven gears, and a shifter adapted to be moved on the main shaft or the countershaft. In the conventional transmission, the shifter is engageable with the drive gears or the driven gears so that the main shaft rotationally drives the countershaft at any one of a plurality of shift positions set by the combination of the drive gears, and the driven gears respectively driven by the drive gears. Such a transmission is disclosed, for example, in Japanese Patent Laid-Open No. 2000-205352.
The shifter in such a gear transmission is axially moved on the main shaft or the countershaft. Accordingly, the drive gears or the driven gears (both of which will be hereinafter generically referred to as “shift gears”) engageable with the shifter have engaging portions axially projecting toward the shifter. However, in a small-diameter gear of the plural shift gears, the radial width of a disc portion of the small-diameter shift gear is small, so that it is difficult to form the engaging portion on the disc portion. Herein, a disc portion of the shift gear corresponds to the annular portion formed between a toothed portion of the shift gear and the main shaft or the countershaft. Accordingly, the engaging portion is usually formed on a large-diameter shift gear meshing with the above small-diameter shift gear.
However, the large-diameter shift gear which includes the engaging portion is increased in weight, and the weight of the shifter may be increased in proportion thereto. Further, in some circumstances, an increased number of shifters must be provided according to the layout of the shift gears, causing an increase in weight of the transmission or an increase in axial size of the transmission.
Further, the first speed drive gear on the main shaft is usually formed integrally with the main shaft by directly working a large-diameter portion of the main shaft into a gear, and the second and higher speed drive gears are mounted from an axial end of the main shaft toward the first speed shift gear. However, in a transmission having two shift clutches (twin clutch), the main shaft is provided by a shaft having a double structure such that the shaft is composed of a first shaft, and a hollow second shaft that is fitted to the outer circumference of the first shaft. In addition, the first speed drive gear is formed integrally with the first shaft. In the case that an axially positioning portion (e.g., projecting portion) is provided on the first shaft so as to prevent the axial movement of the second shaft, there is a possibility that the second and higher speed drive gears cannot be mounted between the first speed drive gear and the axially positioning portion.
It is accordingly an object of the present invention to increase the flexibility of layout of the shift gears and reduce the axial size of the transmission by providing an engaging portion on a specific one of the shift gears. It is another object of the present invention to make it easy to manufacture the specific gear having the engaging portion. It is a further object of the present invention to prevent an increase in axial size of the transmission.